If you are going to aerate, I feel that there really is no need to thatch then unless it is really necessary (half of inch of thatch or greater) Aeration is a process that helps in thatch reduction as well.

Best time for both though is early spring or late summer/early fall. When you dethatch in the spring make sure to put down a pre-emergence weed control and fertilizer. Also remember that the lawn will be under a great deal of stress for 20-30 days. When you aerate throw down some topdressing and if you desire some seed as well